I. Field of the Invention
The invention relates to the formation of fibrous felts such as those intended for thermal and acoustic insulation, and it relates more particularly to a device for improving the distribution of the fibers that are collected on a receiving member.
II. Description of Related Art
The formation of fibers, especially mineral fibers such as glass fibers, results from a fiberizing process consisting in attenuating the material, such as glass, by centrifugation and by the action of high-temperature gas streams.
The fiberizing process commonly used today is what is called the internal centrifugation process. It consists in introducing a stream of attenuable material in the molten state in a spinner, also called a fiberizing dish, rotating at high speed and pierced around its periphery by a very large number of holes via which the material is ejected in the form of filaments owing to the centrifugal force. By means of an annular burner, these filaments are then subjected to the action of an annular stream of high-temperature high-velocity attenuating gas that hugs the wall of the spinner, reducing the diameter of the filaments and converting them into fibers.
Moreover, the attenuating gas stream is generally confined by means of a surrounding cold gas sheath, suitably channeling it in the form of a tubular flow. This gas sheath is produced by a blowing ring that surrounds the annular burner. Being cold, this gas sheath also helps to cool the fibers, the mechanical strength of which is thus probably improved by a thermal quenching effect.
It is also common practice to add an annular inductor beneath the centrifugation device in order to help to maintain thermal equilibrium of the spinner. This inductor heats the bottom of the peripheral band of the spinner, which bottom is heated less by the attenuating gas as it is further away from the annular burner and is cooled by the ambient air.
The fibers formed are entrained by the attenuating gas stream toward a receiving belt generally consisting of a gas-permeable belt onto which the fibers are entangled in the form of a blanket.
To fix the fibers together, a binder is generally sprayed onto the fibers during their path towards the receiving belt. The binder is sprayed for example by means of a ring that surrounds the gas stream and has a plurality of spray holes.
The binder is then cured, for example by a heat treatment beyond the receiving belt.
One of the difficulties encountered in preparing these felts is due to the distribution of the fibers within the entire felt, which distribution is desired to be the most uniform possible. Any irregularity in the distribution may result in a local density lower than the desired density, this generally being corrected during manufacture by increasing the mean density of felt. However, it always desirable to reduce the density of a product in order to make it lighter while obtaining just as good insulation, particularly thermal insulation, performance. The aim is therefore continually to make the distribution of the fibers in the felt as homogeneous as possible on a production line.
A known means for improving the fiber distribution is the use of a device called a “bucket”, as described in patent application FR 2 544 754, which consists of a guiding duct placed in the path of the gas stream beneath the spinner and above the binder spray device. This duct channels the fibers—it undergoes an oscillatory movement in order to direct the flow of fibers alternately from one edge of the fiber-receiving belt to the other.
However, this solution has a tendency to degrade the properties of the fibers by a rubbing effect during their impact on the walls of the bucket device.
Another known means consists in blowing air, toward the tubular gas flow, substantially perpendicularly in order to pass through it.
Patent FR 1 244 530 thus describes two nozzles that are placed above the binder spray device and diametrically opposite the gas flow, and the air jets of which are actuated in turn in order to impress on the veil of fibers a to-and-fro movement when it is being deposited on the receiving belt.
U.S. Pat. No. 4,266,960 shows two devices, which each deliver a flat jet of air arriving at high velocity perpendicular to and in the tubular gas flow, the two devices being placed on either side of the gas flow so that the orientation of the air jets ensures that the tubular flow separates into several divergent flows.
These blowing means therefore constrain the air to be directed so as to be approximately perpendicular to the tubular sheath of fibers, in order to divide the tubular flow and/or to modify its orientation. However, these means run the risk of creating holes in the veil at the moment of impact of the air jet, which in the end does not optimize a uniform distribution of the fibers on the receiving belt.
Furthermore, one of the risks associated with using these means is that they are very difficult to regulate by an operator.
Finally, document FR 2 510 909 discloses a method for improving the fiber distribution, which consists in creating a gas sheath directed so as to hug the tubular veil carrying the fibers around its periphery by enveloping it, the flow direction of this gas sheath in the plane tangential to the tubular veil making an angle to the flow direction of the veil of between 10 and 60°.
The device for implementing this method consists of a blowing ring comprising an annular chamber, the base of which conducts a pressurized gas so as to generate a gas sheath. In one arrangement, the base of the chamber has, in a first embodiment, inclined holes or else, in another embodiment, a continuous slot, the flow direction being provided by inclined fins placed at regular intervals in this slot along the path of the gas sheath.
However, this type of device blows air tangentially to the fibers from the base of the chamber, at an angle of between 10 and 60° relative to the flow of the tubular sheath, and in the region or the end of the region where the fibers are attenuated, thereby preventing adjustment of the distribution, which is independent of the quality of the fiber obtained.